1. Field of the Invention
The invention involves elements for the quantitative or semi-quantiative analysis of aqueous liquids. It particularly concerns analytical elements which contain peroxidase as one of the test reagents.
2. Description of the Related Art
It is well known to perform a quantitative or semiquantitative analysis of an aqueous liquid by contacting that liquid with an analytical element containing a combination of reagents capable of yielding a detectable product in proportion to the concentration of a predetermined analyte in the aqueous liquid. As used herein "reagent" is intended to mean a material that is interactive with the predetermined analyte, a precursor of that analyte, or any material produced during the analysis of that analyte. Such interaction refers to chemical reactivity, catalytic activity, or any other form of chemical or physical interaction that can result in the ultimate production of a change in the element that is detectable by suitable measurement of radiant energy, usually in the visible light range of the spectrum.
One general group of particularly useful analytical elements utilizes an enzymatic assay wherein the predetermined analyte, upon contact with the analytical element, reacts with oxygen in the presence of a suitable enzyme contained in the element to produce a peroxide in proportion to the concentration of the predetermined analyte in the aqueous liquid being analyzed. A detectable product is then yielded by the reaction of this peroxide with an indicator composition in the presence of peroxidase, both of which are also contained in the analytical element. The detectable product is formed within the element in direct proportion to the peroxide present and thus, also in proportion to the concentration of the predetermined analyte in the aqueous liquid. Elements and analyses of this type are described for example in U.S. Pat. No. 3,992,158 and in a copending U.S. application by Barbara J. Bruschi, Ser. No. 712,972, filed Aug. 9, 1976, U.S. Pat. No. 4,089,747, both of which are incorporated herein by reference. Such elements usually contain the test reagents described above, including peroxidase, in a carrier material that is permeable to aqueous liquids and to the predetermined analyte of choice. The term "predetermined analyte" is meant to refer to the substance whose concentration is intended to be measured during the analysis. The carrier may be the main material forming the element or just one layer in a multi-layered element. Its permeability assures that the aqueous liquid and predetermined analyte will come into contact with each of the test reagents contained in the carrier when such aqueous liquid is brought into contact with the carrier.
One of the advantages of such analytical elements is that analyses can be performed quickly and reliably by persons with little technical training and no access to a "wet" chemical laboratory. A doctor, for instance, might find it extremely useful to store a supply of various elements of this type for use in "on-the-spot" analyses of body fluids such as urine or blood serum as an aid to diagnoses.
Unfortunately, because of the carrier's necessary permeability to aqueous liquids, the test reagents contained in the carrier may deteriorate during significant periods of storage and thus deleteriously affect the accuracy and reliability of the analysis. For example, exposure to air and moisture may adversely affect the ability of peroxidase contained in a test element to catalyze the oxidation of an indicator composition by a peroxide, thus preventing the formation of detectable product accurately and consistently in proportion to the concentration of predetermined analyte.
In order to improve the stability of reactants during periods of storage it has been thought desirable to construct the carrier totally from a hydrophobic material to block moisture and air from penetrating to the test reagents, such as in U.S. Pat. No. 3,630,957. Unfortunately, such carriers are not sufficiently permeable to aqueous liquids and therefore would not be useful for analyses which require that aqueous liquids be able to penetrate to the test reagents.
U.S. Pat. Nos. 3,212,855 and 3,598,704, suggest adding water-soluble polymers or hydrophilic colloids to a bibulous carrier to keep test reagents physically separated and to help prevent deterioration of such reagents because of the effects of moisture. Although such polymers may slow the rate of penetration of moisture and air to test reagents, since they are water soluble polymers, it is clear that moisture will eventually penetrate them. The protection they afford is therefore limited.
U.S. Pat. No. 3,616,251, suggests imbedding the test reagents into the surface of a carrier material comprising a hydrophobic polymer by using an organic solvent, but the stated purpose is to prevent the reagents from washing away when contacted with aqueous liquids, and it appears that the reagents would still be susceptible to the deteriorative chemical effects of moisture and air.
A different problem may additionally occur when an analytical element comprises a carrier permeable to aqueous liquids which is coated on a hydrophobic support material. It involves unwanted curl. This is caused by absorption and evaporation of moisture in the carrier resulting in swelling and shrinking of the carrier relative to the support material, which does not absorb or evaporate moisture. This problem is well known in the photographic industry, where materials such as gelatin are coated on hydrophobic support materials to form photographic films. U.S. Pat. No. 3,459,790, suggests the inclusion of certain hydrophobic polymers in gelatin layers of photographic films to alleviate this problem.
In light of the discussion above it would be desirable if an analytical element could be devised which comprises a carrier that is permeable to aqueous liquids and contains test reagents such as peroxidase, but the analytical element does not suffer from the above-mentioned problem of instability of reactants such as peroxidase during periods of storage. The present invention provides such an element.